Photonic crystals (PCs) are materials having structure that includes a periodic modulation in their refractive index (Yablonovitch, Phys. Rev. Lett., 58:2059, 1987), giving rise to a photonic band gap or stop gap, in which electromagnetic waves within a certain stop band wavelength range may be mostly or totally reflected. The wavelengths of the stop band may be dependent on the distance between the periodic modulations in the crystal. The reflected stop band wavelengths may appear in the reflectance spectrum as a reflectance peak known as a Bragg peak. A photonic crystal may have a one-, two-, or three-dimensional periodic structure.
Slight changes in the refractive index and/or structure of a photonic crystal material may result in optically detectable changes in the reflectance spectrum. Controlled changes to the reflectance spectrum may be referred to as “tuning.” This may be useful where the reflected light is in the visible range, for example allowing for detectable changes in color if the refractive index and/or lattice spacing is modulated. In some examples, mechanically deformable materials, such as polymers, may be incorporated into photonic crystal materials, such that the PC may be made responsive to mechanical stimulation, such as compression or stretching. Examples of such an application may be found in PCT Publication No. WO2008/098339, and in PCT Publication No. WO2006/097173, which are herein incorporated by reference in their entireties.
FIG. 1 illustrates some examples of how a mechanically deformable photonic crystal material may be deformed to change its lattice constant along at least one direction. In this example, the photonic crystal material may have an inverse opal structure, including a plurality of ordered voids in a deformable polymer matrix. Similar principles may apply to materials having periodicity in only 2 or 1 dimension, such as for hollow rods in a polymer slab, or for a material consisting of alternating planar layers (which may be also referred to as a Bragg Stack). As shown, an example mechanically deformable photonic crystal material may be stretched or compressed along one of its dimensions (i.e., height, width or length). Such mechanical deformation may cause a change in the lattice structure of the material, resulting in a change in the reflectance spectrum of the material. In some examples, such mechanical deformation may be at least partially reversible and/or repeatable.